> However, to notice this displacement, you'd need to be looking at
> the display, in which case you'd have to factor in the eye's persistence
> of vision - at that dimness and at 200Hz

"The dark adapted eye can see a single photon" :-).

FWIW while that is true almost by definition, it's misleading.
Can and will are <> at that level.
While you can safely say "you will be able to see an LED operated at 1
mA" and only have pedants disagree, the single photon is hit and miss
where hit < to << miss.
ie it has to strike a receptor and the receptor has to fire (as is
their job to do when struck with single photons) and you brain has to
be attuned to what you are looking for.

All that said, it's a stunningly amazing capability. Which may or may
not have high relevance to seeing 50 uS LED pulses at 1% duty cycle.

________________

Short:

If it's a typical small LCD backlight then the use of a PWM derived DC
driven display with a linear pass element and current sensing  seems
dandy.
If it's a 100 W backlight  this may produce excessive heat.

Go on volume up:

If hardware choice were free (which may be) and pins plentiful and
power budget irrelevant you could do a two stage dim. Stage on is say
4 bits and is filtered to DC. Stage two is say 4 more bits off the
first DC level. 8 bits with 2 x 4 bit slowwwwwwwwwwwwwwww PWM
channels. 8 bit equivalent. More or less. See below re display
response to decreasing DC levels.

Or, while the spec says stupidlow <=3D PWM freq <=3D stupidhigh  or
whatever, do they actually say what the engineer has to or may do with
the PWM twixt PWM pin and LED? If not then filtering to ~~~ DC would
remove aforementioned spurious effects. Note that LED response to DC
filtered PWM is very very much not the same as to the PWM source.
However, sufficient unto the software is the functionality thereof.

If you can make your feedback work on LED current rather than PWM %
(which is trivially easy if they let you add an opamp and a
transistor)(sense resistor current matches DC filtered PWM level) then
you are back to linear response with DC drive.

Looking back now at spec door after the solution has bolted the above
seems to match items 5 and 4 of your spec and power level was not
mentioned (if eye/brain are cooperating). If it's a 100 W backlight
this may produce excessive heat. If it's a typical small LCD backlight
then the use of a PWM derived DC driven display with a linear pass
element  and current sensing  seems dandy.



            Russell
--=20
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